Sometimes when you use iTunes to sync your iPhone or iPod Touch, you might get a error at the end of the sync which says that certain apps could not be synched to the phone because of an unknown error 0xE8008001 occurred because of which the apps could not be installed on the iPhone or iPod Touch.
The error occurs when you try to directly install .IPA files on your iPhone or iPod Touch and because the Mobile Installation through iTunes does not support this.
In order to get rid of this error you will first of all have to jailbreak iPhone or jailbreak iPod Touch and then install a Cydia app called AppSync, which patches the Mobile Installation files and allows you to get rid of the above error thrown by iTunes. This problem also occurs on the recently released iOS 4.
If you do not know how to install the AppSync app, take a look at how to install AppSync on iPhone and iPod Touch.